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September 25, 2019 – Hot and dry weather with breezy northerly winds will continue critical fire weather conditions today. A significant cool down is expected late this week with a chance of showers and high elevation snow.

Discussion

Skies remain mostly clear other than a batch of cirrus moving over the eastern Pacific ridge southeastward into NorCal. Surface pressure gradients are a little tighter compared to 24 hours ago (MFR-SAC is around 12 mbs and RNO-SAC is 8.5 mbs) and winds are a little stronger too with local gusts of 25-50 mph currently across the west slopes of the northern Sierra and foothills. Local gusts of 15-30 mph are present along the west side of the Sacramento Valley into the Coast Range.

Current temperatures are milder across most of the area compared to early Tuesday, though areas where winds are light are actually a little cooler. Readings range from the lower 40s in the mountain valleys to the lower to mid 80s across breezier portions of the Sacramento Valley.

Critical fire weather concerns will continue into this evening. The strongest winds will be over the exposed ridges and northeast oriented canyons of the Sierra Nevada and foothills this morning, then in the Sacramento Valley into this evening.

Hot weather continues today with warmest Valley highs expected to be a little warmer than Tuesday. Some cooling will be possible Thursday as short-waves moving through the Pacific Northwest flatten the ridge and induce a return of onshore flow. Stronger cooling will begins Friday with the approach of the deep trough along the West Coast. Shower chances begin in the northern mountains Friday night and spread southeast through the northern Sierra on Saturday.

Extended Discussion (Sunday through Wednesday)

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A cool upper low originating from northwestern Canada will drop down into the region Sunday. This will bring unsettled weather with unseasonably cool temperatures, upwards of 15 to 25 degrees below normal. The origin over land will limit the amount of precipitation seen with this system, so it is not expected to be especially wet.

Latest GEFs and ENS ensembles show upper low has continued eastward trend, decreasing the potential for precipitation over the Coastal Range and Valley. Chances of rain and high mountain snow are focused over the southern Cascades, northern Sierra and the foothills. This potential spreads into the northern Coastal Range Sunday afternoon and night as another reinforcing wave drops down. This will bring the Valley a slight chance of rain showers Sunday night into Monday.

As colder air is ushered into the CWA over the weekend snow levels will lower to 5500 to 6500 feet with some accumulations possible over higher terrain. Several inches of snow are possible at pass levels, though this will be spread out over a few days. This could cause minor travel impacts. By Tuesday, it appears the low will transition shift eastward with dry weather over the area.

Northerly winds could develop Tuesday and Wednesday, bringing a return of elevated fire weather concerns.